Simple GPU based dithering

src/gpu/SkGr.cpp

Index: src/gpu/SkGr.cpp
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index 8e7cea64d4a5b306184eee943c1afb9c33431832..286452c10d67e4074cdaa8d67fd1015513d4587c 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -13,6 +13,7 @@
 #include "SkPixelRef.h"
 #include "GrResourceCache.h"
 #include "GrGpu.h"
+#include "effects/GrDitherEffect.h"
 #include "GrDrawTargetCaps.h"
 
 #ifndef SK_IGNORE_ETC1_SUPPORT
@@ -460,6 +461,29 @@ void SkPaint2GrPaintNoShader(GrContext* context, const SkPaint& skPaint, GrColor
             }
         }
     }
+
+#ifndef SK_IGNORE_GPU_DITHER
+    // If the dither flag is set, then we need to see if the underlying context
+    // supports it. If not, then install a dither effect.
+    if (skPaint.isDither()) {
+        // What are we rendering into?
+        const GrRenderTarget *target = context->getRenderTarget();
+        SkASSERT(NULL != target);
+
+        // Suspect the dithering flag has no effect on these configs, otherwise
+        // fall back on setting the appropriate state.
+        if (target->config() == kRGBA_8888_GrPixelConfig ||
+            target->config() == kBGRA_8888_GrPixelConfig) {
+            // The dither flag is set and the target is likely
+            // not going to be dithered by the GPU.
+            SkAutoTUnref<GrEffectRef> effect(GrDitherEffect::Create());
+            if (NULL != effect.get()) {
+                grPaint->addColorEffect(effect);
+                grPaint->setDither(false);
+            }
+        }
+    }
+#endif
 }
 
 /**
@@ -494,19 +518,22 @@ void SkPaint2GrPaintShader(GrContext* context, const SkPaint& skPaint,
     // SkShader::asNewEffect() may do offscreen rendering. Save off the current RT, clip, and
     // matrix. We don't reset the matrix on the context because SkShader::asNewEffect may use
     // GrContext::getMatrix() to know the transformation from local coords to device space.
-    GrContext::AutoRenderTarget art(context, NULL);
-    GrContext::AutoClip ac(context, GrContext::AutoClip::kWideOpen_InitialClip);
-    AutoMatrix am(context);
-
-    // setup the shader as the first color effect on the paint
-    // the default grColor is the paint's color
     GrColor grColor = SkColor2GrColor(skPaint.getColor());
-    GrEffectRef* grEffect = NULL;
-    if (shader->asNewEffect(context, skPaint, NULL, &grColor, &grEffect) && NULL != grEffect) {
-        SkAutoTUnref<GrEffectRef> effect(grEffect);
-        grPaint->addColorEffect(effect);
-        constantColor = false;
+    {

[bsalomon, 2014/06/19 13:33:27]

Can you add a comment that this block is here to restore the context state after
asNewEffect is called. I'm just concerned that we'll forget why this was done
and remove what appear to be unnecessary braces.

[krajcevski, 2014/06/19 13:51:43]

Done.

+        GrContext::AutoRenderTarget art(context, NULL);
+        GrContext::AutoClip ac(context, GrContext::AutoClip::kWideOpen_InitialClip);
+        AutoMatrix am(context);
+
+        // setup the shader as the first color effect on the paint
+        // the default grColor is the paint's color
+        GrEffectRef* grEffect = NULL;
+        if (shader->asNewEffect(context, skPaint, NULL, &grColor, &grEffect) && NULL != grEffect) {
+            SkAutoTUnref<GrEffectRef> effect(grEffect);
+            grPaint->addColorEffect(effect);
+            constantColor = false;
+        }
     }
+
     // The grcolor is automatically set when calling asneweffect.
     // If the shader can be seen as an effect it returns true and adds its effect to the grpaint.
     SkPaint2GrPaintNoShader(context, skPaint, grColor, constantColor, grPaint);